U.S. patent number 8,976,908 [Application Number 13/920,040] was granted by the patent office on 2015-03-10 for channel estimation for a wlan through increased use of preamble.
This patent grant is currently assigned to Marvell International Ltd.. The grantee listed for this patent is Marvell International Ltd.. Invention is credited to Rohit U. Nabar.
United States Patent |
8,976,908 |
Nabar |
March 10, 2015 |
Channel estimation for a WLAN through increased use of preamble
Abstract
A system including a receiver to receive, over a wireless
channel, a packet including a preamble portion that includes a
plurality of training fields and a first signal field and a
controller to generate a first estimate of the wireless channel
based on one or more of the plurality of training fields, and
decode the first signal field based on the first estimate.
Subsequent to decoding the first signal field, the controller
re-encodes the first signal field to generate a second signal
field. The controller verifies whether the first signal field was
properly decoded by comparing the second signal field to the first
signal field prior to being decoded. In response to the first
signal field being correctly decoded, the controller generates a
second estimate of the wireless channel based on the first signal
field as decoded. The second estimate is usable to refine the first
estimate.
Inventors: |
Nabar; Rohit U. (Sunnyvale,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Marvell International Ltd. |
Hamilton |
N/A |
BM |
|
|
Assignee: |
Marvell International Ltd.
(Hamilton, BM)
|
Family
ID: |
45694509 |
Appl.
No.: |
13/920,040 |
Filed: |
June 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13405904 |
Feb 27, 2012 |
8467481 |
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11895164 |
Feb 28, 2012 |
8126090 |
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60823975 |
Aug 30, 2006 |
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Current U.S.
Class: |
375/340;
375/316 |
Current CPC
Class: |
H04L
25/0236 (20130101) |
Current International
Class: |
H04L
27/00 (20060101) |
Field of
Search: |
;375/316,340,262,260,365,368 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
ANSI/IEEE Std 802.11, 1999 Edition; Information
technology--Telecommunications and information exchange between
systems--Local and metropolitan area networks--Specific
requirements--Part 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications; LAN/MAN Standards Committee of
the IEEE Computer Society; 531 pages. cited by applicant .
802.11n; IEEE P802.11-04/0889r6; Wireless LANs, TGn Sync Proposal
Technical Specification; May 2005; 131 pages. cited by
applicant.
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Primary Examiner: Bayard; Emmanual
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation of U.S. patent application Ser. No.
13/405,904, filed on Feb. 27, 2012, which is a continuation of U.S.
patent application Ser. No. 11/895,164 (now U.S. Pat. No.
8,126,090), filed on Aug. 23, 2007, which claims the benefit of
U.S. Provisional Patent Application No. 60/823,975, filed on Aug.
30, 2006. The entire disclosures of the applications referenced
above are incorporated herein by reference.
Claims
What is claimed is:
1. A system, comprising: a receiver configured to receive, over a
wireless channel, a packet including a preamble portion, wherein
the preamble portion includes (i) a plurality of training fields
and (ii) a first signal field; and a controller configured to
generate a first estimate of the wireless channel based on one or
more of the plurality of training fields, decode the first signal
field based on the first estimate of the wireless channel,
subsequent to decoding the first signal field, re-encode the first
signal field to generate a second signal field, verify whether the
first signal field was properly decoded by comparing (i) the second
signal field to (ii) the first signal field prior to being decoded,
and in response to the first signal field being determined to have
been correctly decoded based on the comparing of (i) the second
signal field to (ii) the first signal field prior to being decoded,
generate a second estimate of the wireless channel based on the
first signal field as decoded, wherein the second estimate of the
wireless channel is usable to refine the first estimate of the
wireless channel.
2. The system of claim 1, wherein the first signal field is located
between two of the plurality of training fields.
3. The system of claim 1, wherein the controller is configured to
generate a third estimate of the wireless channel based on (i) the
first estimate of the wireless channel and (ii) the second estimate
of the wireless channel, and wherein the third estimate of the
wireless channel corresponds to a refinement of the first estimate
of the wireless channel.
4. The system of claim 3, wherein the controller is configured to
decode data in a data portion of the packet based on the third
estimate of the wireless channel.
5. The system of claim 3, wherein the controller is further
configured to: initially decode data in a data portion of the
packet based on the first estimate of the wireless channel; and
subsequent to the second estimate of the wireless channel being
generated, decode data in the data portion of the packet based on
both of (i) the first estimate of the wireless channel and (ii) the
second estimate of the wireless channel.
6. The system of claim 1, wherein: the preamble portion further
comprises a field between the first signal field and one of the
plurality of training fields, and the controller is configured to
generate a third estimate of the wireless channel based on (i) the
first estimate of the wireless channel, (ii) the second estimate of
the wireless channel, and (iii) the field.
7. A system, comprising: a receiver configured to, over a wireless
channel, receive a packet including a preamble portion, wherein the
preamble portion includes (i) a first training field and (ii) a
second training field; and a controller configured to generate a
first estimate of the wireless channel based on the first training
field, generate a second estimate of the wireless channel based on
a first portion of the second training field, and generate a third
estimate of the wireless channel based on (i) the first estimate of
the wireless channel and (ii) the second estimate of the wireless
channel.
8. The system of claim 7, wherein the first training field is
longer than the second training field.
9. The system of claim 7, wherein the second training field
includes a Legacy Short Training Field used in Greenfield mode.
10. The system of claim 7, wherein the controller is configured to
use a second portion of the second training field for
synchronization or to adjust a gain of the receiver.
11. A method, comprising: receiving, at a receiver, over a wireless
channel, a packet including a preamble portion, wherein the
preamble portion includes (i) a plurality of training fields and
(ii) a first signal field; generate a first estimate of the
wireless channel based on one or more of the plurality of training
fields, decoding the first signal field based on the first estimate
of the wireless channel; subsequent to decoding the first signal
field, re-encoding the first signal field to generate a second
signal field; verifying whether the first signal field was properly
decoded by comparing (i) the second signal field to (ii) the first
signal field prior to being decoded; and in response to the first
signal field being determined to have been correctly decoded based
on the comparing of (i) the second signal field to (ii) the first
signal field prior to being decoded, generating a second estimate
of the wireless channel based on the first signal field as decoded,
wherein the second estimate of the wireless channel is usable to
refine the first estimate of the wireless channel.
12. The method of claim 11, wherein the first signal field is
located between two of the plurality of training fields.
13. The method of claim 11, further comprising generating a third
estimate of the wireless channel based on (i) the first estimate of
the wireless channel and (ii) the second estimate of the wireless
channel, and wherein the third estimate of the wireless channel
corresponds to a refinement of the first estimate of the wireless
channel.
14. The method of claim 13, further comprising decoding data in a
data portion of the packet based on the third estimate of the
wireless channel.
15. The method of claim 13, further comprising: initially decoding
data in a data portion of the packet based on the first estimate of
the wireless channel; and subsequent to the second estimate of the
wireless channel being generated, decoding data in the data portion
of the packet based on both of (i) the first estimate of the
wireless channel and (ii) the second estimate of the wireless
channel.
16. The method of claim 11, wherein the preamble portion further
comprises a field between the first signal field and one of the
plurality of training fields, the method further comprising:
generating a third estimate of the wireless channel based on (i)
the first estimate of the wireless channel, (ii) the second
estimate of the wireless channel, and (iii) the field.
17. A method, comprising: receiving, at a receiver, over a wireless
channel, a packet including a preamble portion, wherein the
preamble portion includes (i) a first training field and (ii) a
second training field; generating a first estimate of the wireless
channel based on the first training field; generating a second
estimate of the wireless channel based on a first portion of the
second training field; and generating a third estimate of the
wireless channel based on (i) the first estimate of the wireless
channel and (ii) the second estimate of the wireless channel.
18. The method of claim 17, wherein the first training field is
longer than the second training field.
19. The method of claim 17, wherein the second training field
includes a Legacy Short Training Field used in Greenfield mode.
20. The method of claim 17, further comprising using a second
portion of the second training field for synchronization or to
adjust a gain of the receiver.
Description
BACKGROUND
A commonplace type of WLAN system is the 802.11 IEEE (Institute of
Electrical and Electronics Engineers) standard. Up until 2004,
802.11 wireless devices had a single antenna (some devices had two
antennas, but there was only one set of components to process the
signal, or RF chain). 802.11 task group N (TGn) has recently
proposed an 802.11n standard that has the goal of increasing the
peak data throughput transmitted by a wireless
multiple-input/multiple-output (MIMO) device to 100 Mbps. The basis
of MIMO operation is to provide 11n devices with multiple radio
interfaces to allow the devices to send data on different channels
at the same time in order to achieve greater transmit/receive data
rates than the pre-11n devices.
In a WLAN, information is transmitted between a transmitter and a
receiver through the transmission of encoded data packets. These
data packets are then decoded at the receiver based on channel
state information (CSI). The more accurate the CSI, the more
accurate will be the decoding precision, and hence lower the packet
error rate. Because a WLAN may suffer from impairments such as
delay spread, thermal noise, and bandwidth fluctuations due to a
shared medium with multiple users in a mobile environment, a
perfect CSI is never known to the receiver. Instead, the receiver
must rely on channel estimation. In a WLAN, such as 802.11n,
channel estimation is performed at the receiver through training
sequences that are transmitted prior to the data. The training
sequences are pre-determined and known to the receiver, enabling
the receiver to estimate the CSI. For example, in a WLAN 802.11n
system, the packet structure includes a preamble that is followed
by the data. The preamble includes several types of fields,
including several training fields.
Although the other types of fields in the preamble include a
variety of information, currently, channel estimation performed at
the receiver is restricted to use of only the training fields of
the packet preamble. Accordingly, by limiting the channel
estimation to only use of the training fields of the preamble, the
accuracy of the channel estimation may be likewise limited.
BRIEF SUMMARY
The present invention provides a method and system for improving
channel estimation in a wireless device. Aspects of the exemplary
embodiment include receiving a data packet wirelessly transmitted
from a transmitter, the packet including a preamble portion and a
data portion containing data, wherein the preamble portion includes
at least one training field and a second field; performing a first
channel estimation using the training field; and using the second
field to refine the first channel estimation.
According to the method and system disclosed herein, receiving
wireless devices are no longer limited to using only the training
fields of packet preambles for calculation of channel estimation,
thereby improving the accuracy of the channel estimate, and
consequently the accuracy of the decoding of the data.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a block diagram illustrating an exemplary wireless
communication system.
FIG. 2 is a flow diagram illustrating a process performed by the
driver for improving channel estimation in a wireless device in
accordance with the exemplary embodiment.
FIG. 3 shows an exemplary packet format as proposed by the WLAN
802.11n system in a mode of operation referred to as Greenfield
mode.
FIG. 4 is a flow diagram illustrating the process performed by the
driver for providing an improved the channel estimate in further
detail according to one exemplary embodiment.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to improving the accuracy of channel
estimates in a WLAN receiver through increased use of packet
preambles. The following description is presented to enable one of
ordinary skill in the art to make and use the invention and is
provided in the context of a patent application and its
requirements. Various modifications to the preferred embodiments
and the generic principles and features described herein will be
readily apparent to those skilled in the art. Thus, the present
invention is not intended to be limited to the embodiments shown,
but is to be accorded the widest scope consistent with the
principles and features described herein.
The preferred embodiment provides a method and system for
increasing the accuracy of channel estimates for use in wireless
communication receiver devices. According to the exemplary
embodiments disclosed herein, estimating the channel at the
receiver when a transmitted packet is received is not restricted
only to the training fields of the packet preamble. Instead,
additional portions of the preamble may be used to improve the
quality of the channel estimation. By increasing the accuracy of
the channel estimation, the accuracy of the receiver's decoding is
increased, which in turn, may lower the packet error rate.
The exemplary embodiments will be described in terms of improved
channel estimation using Greenfield access mode in IEEE 802.11n
WLAN systems. However, one with ordinary skill in the art will
readily recognize that the exemplary embodiments may be used with
other types of access modes and other types of wireless
communication systems in which packets are transmitted with
preambles having various fields besides a training field. The
exemplary embodiments will also be described in the context of
particular methods having certain steps. However, the method and
system operate effectively for other methods having different
and/or additional steps not inconsistent with the exemplary
embodiments.
FIG. 1 is a block diagram illustrating an exemplary wireless
communication system. The wireless communication system 10 includes
a wireless device 12 that communicates with another wireless
device, for example, an access point (AP) 14. In one exemplary
embodiment, the wireless device 12 comprises a MIMO device. In
addition, the wireless device 12 may be a network device or client
station (STA) used in a desktop/portable computer for
communication. The access point 14 is the device that connects
multiple wireless devices 12 together to form a wireless network
and permits wireless devices 12 to communicate over the network or
to each other. An example access point 14 is a router that has a
broadband network connection. Several access points 14 can link
together to form a larger network that allows roaming. The wireless
device 12 and the access point 14 communicate through the
transmission of signals in the form of streams of encoded data
packets 15 over one or more radio frequency (RF) channels. In the
context of referring to different layers in a particular protocol
reference model, packets 15 may sometimes be referred to as
frames.
Although both the wireless device 12 and the access point 14 are
capable of both transmitting and receiving signals, for the
purposes of this disclosure, the wireless device 12 will be
described in terms of being the receiver that receives data packets
15 transmitted from the access point 14.
The wireless device 12 includes one or more independent radio
interfaces 16a and 16n (commonly referred to as radio interfaces
16) for processing one or more data streams, a controller 18
coupled to the radio interfaces 16, a memory 20 coupled to the
controller 18, and a bus interface unit 22 coupled to the
controller 18 and to the memory 20 for transmitting data to a host
24 over a host system bus.
The radio interfaces 16 are independent from each other because
each radio interface 16 has its own antenna and RF chain. Each RF
chain and its corresponding antenna are responsible for
transmitting and processing a data stream. A single frame of data
can be broken up and multiplexed across multiple data streams and
reassembled at the receiver, which may have the benefits of
resolving multipath interference and improving the quality of the
received signal. In one embodiment, the devices in the wireless
communication system 10 may have a different number of receive
antennas than transmit antennas. Each of the radio interfaces 16
may utilize 1 to N radio channels. Each of the channels may be used
simultaneously for data transmission. The wireless device 12 may
also include multiple operating modes including one or more data
modes corresponding to the number of available radio interfaces
16.
The driver 26 is software or firmware that controls the radio
interfaces 16 and can process the data if needed. The driver 26 is
executed by the controller 18. The controller 18 may comprise an
ASIC, a DSP or other type of processor. The memory 20 stores the
incoming and outgoing data packets 15 and any other data needed by
the driver 26. The bus interface unit 22 transfers data between the
host system 24, and the controller 18 and the memory 20.
In operation, when the wireless device 12 receives one or more data
packets 15, the driver 26 decodes the data packets 15 based on
channel state information (CSI). Because an exact CSI cannot be
determined by the receiving wireless device 12, the driver 26
conventionally performs channel estimation through training
sequences that are transmitted in the data packets 15 prior to the
actual data within the packets 15.
FIG. 2 is a flow diagram illustrating a process performed by the
driver 26 for improving channel estimation in a wireless device 12
in accordance with the exemplary embodiment. Referring to both
FIGS. 1 and 3, the process begins when the wireless device 12
receives a signal comprising a data packet 15 wirelessly
transmitted from a transmitter, such as access point 14 (block
200). Depending on the type of wireless protocol and the device's
mode of operation within that protocol, the format of the data
packets 15 may vary. However, typically, the packet format includes
a preamble portion followed by a data portion containing actual
data.
The preamble contains information essential to establish a link
with the transmitter via a radio frequency (RF) channel through
gain control, synchronization, channel estimation, etc. In some
systems, the same preamble may be transmitted on all the device
antennas, but with small time shifts relative to the others. The
preamble may include several types of fields, including one or more
training fields used traditionally for estimation, and at least one
other type of field. The preamble in most modes of the WLAN 802.11
standard, for example, includes both short and long training
fields, and a signal field.
FIG. 3 shows an exemplary packet format as proposed by the WLAN
802.11n system in a specific mode of operation referred to as
Greenfield mode. A packet 300 in Greenfield mode includes a
preamble 302 preceding a data field 304 (which contains a physical
sub-layer data unit). The preamble 302 includes a Legacy Short
Training Field (L-STF) 306; a High Throughput Long Training Field
(HT-LTF) 308a; a High Throughput Signal Field (HT-SIG) 310; and
several (e.g., 4) additional High Throughput Long Training Fields
(HT-LTF's) 308b. Other fields not shown that may be utilized in the
preamble in other modes of WLAN 802.11n include a Legacy Long
Training Field (L-LFT), a Legacy Signal Field (L-SIG), and a High
Throughput Short Training Field (HT-STF).
The Legacy Short Training Field 306 is typically used by the
receiver for synchronization and automatic gain control (AGC). The
HT-Signal Field 310 provides information that helps to decode the
data in the data field 304, such as the modulation, coding rate,
and number of spatial streams.
The training portion of the preamble 302 comprising the HT-Long
Training Fields 308a and 308b (collectively referred to as training
fields 308), contains the training sequences traditionally used to
obtain CSI through channel estimation at the receiver, which is
then used to decode the data field 304. As described above,
traditionally channel estimation has in the past been restricted to
use of only the training portion of the preamble 302, i.e., the
training fields 308.
Referring again to FIG. 2, the driver 26 also performs channel
estimation using one or more of the training fields 308 of the
preamble 302 (block 202). However, in accordance with an exemplary
embodiment, the driver 26 also uses a least one other field of the
preamble 302 to refine the channel estimation (block 204).
According to the exemplary embodiment, the other field of the
preamble 302 used to refine the initial channel estimates is the
HT-SIG (hereinafter signal field) 310. The signal field 310
contains information necessary for enabling the receiving wireless
device 12 to correctly decode the data 304. The signal field 310 is
particularly suitable for channel estimation because the signal
field 310 is encoded (i.e., digitally modulated) with a high
strength code and can very reliably be decoded at the receiver over
a wide range of channel conditions. More specifically, the signal
field 310 is encoded using Phase-shift keying (PSK) and also
includes a cycle redundancy check (CRC). Even if there are errors
in the channel estimate using the training fields 308, the signal
field 310 is almost always error-free.
According to the exemplary embodiment, once the receiver decodes
the signal field 310 correctly, the receiver can re-encode the
signal field 310 to reconstruct the signal that was transmitted,
and in effect, provide a longer training sequence. Once the signal
field 310 has been successfully reconstructed, the receiver can use
the signal field 310 to compute a second channel estimation that
may be used to refine the initial channel estimation computed from
the long training fields. According to one embodiment, the initial
channel estimation may be refined by combining (e.g., averaging)
the initial channel estimation with the second channel estimation,
providing the refined channel estimation that is used to decode the
data 304.
FIG. 4 is a flow diagram illustrating the process performed by the
driver 26 for providing an improved the channel estimation in
further detail according to one exemplary embodiment. The process
begins by calculating the initial channel estimation based on one
or more training fields 308 of the preamble 302 (block 400). In one
embodiment, the initial channel estimation may be determined based
on only the first long training field (HT-LTF1) 308a. The initial
channel estimation may be calculated from the training field(s) 308
using conventional techniques, the details of which are well-known
to those of ordinary skill in the art.
After calculating the initial channel estimation, the driver 26
uses the initial channel estimation to decode parameters of the
signal field 310 (block 402). The decoded parameters are re-encoded
(using the same type of encoding originally used to encode the
signal field 310) to provide a reconstructed signal field (block
404). The decoding is validated by comparing the signal field 310
with the reconstructed signal field (block 406). If validated, the
driver 26 calculates a second channel estimation based on the
parameters of decoded signal field 310 (block 408). In one
embodiment, the second channel estimation is calculated using the
same conventional techniques used to calculate the initial channel
estimation. The driver 26 then averages the initial channel
estimation with the second channel estimation to obtain an improved
channel estimate (block 410). In another embodiment, the initial
channel estimation may be refined by the second channel estimation
using a weighted average. The improved channel estimate is then
used to e decode the data 304 in the packet 300 (block 412).
Because the averaged channel estimation is based on both the long
training fields 308 in addition to another field of the preamble
302, the averaged channel estimation should increase the accuracy
of CSI estimation at the receiver. The more accurate the CSI
estimation, the more accurate the decoding of the data 304, and
hence lower the packet error rate. Accordingly, improving channel
estimation by performing the estimates using the training fields
308 as well as another field of the preamble 302 accordance with
the exemplary embodiments may result in a decoding performance gain
of approximately 1 dB.
In the process above, the signal field 308 is encoded and then
re-encoded to obtain the averaged channel estimation prior to the
driver 26 decoding the data 304. According to an alternative
embodiment, the signal field 310 is used even as the data 304 is
being processed. In this embodiment, the initial channel estimation
is first obtained and the signal field 308 is then decoded. After
the signal field 308 is decoded, the decoding of the data 304 is
commenced. Any free cycles encountered during the decoding of the
data 304 is then used to obtain channel estimations based on the
decoded signal field, which together with the initial channel
estimation are used to provide the refined channel estimation. The
refined channel estimation is then used for the decoding of the
remaining data 304, thereby increasing accuracy of the channel
estimation as the data 304 is being processed.
In another embodiment, another preamble field other than the signal
field 310 may be used to augment the channel estimation obtained
from the training fields 308. For example, a short training field
may be used to improve channel estimation, such as the Legacy Short
Training Field (L-SFT) 306 used in Greenfield mode. Typically, the
short training field is used to achieve standardization or to
adjust the gain. However, the short training field could be used in
exemplary embodiments by using half of the short training field to
achieve synchronization or to adjust gain, and then using the other
half to obtain the second channel estimation.
In yet another embodiment, some mixed operating mode packet formats
may include a guard area in-between the signal field 310 and the
long training field 308 in the preamble. Typically, a modulator
simply removes the guard area. However, the guard area contains
just a copy of the last few samples of the long training field 308.
Accordingly, additional use can be made of the guard area along
with the signal field 310 in addition to the long training field to
generate the refined channel estimation.
A method and system for improving channel estimation for WLANs
through increased use of preamble has been disclosed. Exemplary
embodiments have been provided that describe how the preamble 302
may be additionally leveraged beyond use of the training fields for
improved CSI estimation at the receiver. The embodiments are not
limited to the WLAN Greenfield mode of operation or limited to
using specifically the signal field 310 for improved channel
estimation. In general, additional use of the preamble (not
restricting CSI estimation to the LTF), under appropriate
conditions, can improve CSI estimation accuracy considerably and
reduce the decoding error.
The present invention has been described in accordance with the
embodiments shown, and one of ordinary skill in the art will
readily recognize that there could be variations to the
embodiments, and any variations would be within the spirit and
scope of the present invention. For example, the present invention
can be implemented using hardware, software, a computer readable
medium containing program instructions, or a combination thereof.
Software written according to the present invention is to be either
stored in some form of computer-readable medium such as memory or
CD-ROM, or is to be transmitted over a network, and is to be
executed by a processor. Consequently, a computer-readable medium
is intended to include a computer readable signal, which may be,
for example, transmitted over a network. Accordingly, many
modifications may be made by one of ordinary skill in the art
without departing from the spirit and scope of the appended
claims.
* * * * *